I have thought that if there was a sump hole in the floor of the furnace where you were confident that glass would flow to in the event of a leaking crucible then the hole could be filled with frax and a thermocouple could be placed in this hole and monitored...This method may not help much in a catastrophic loss but would work against slow insidious leaks that might go unnoticed for a long time.

I want to tie this back to an old thread Electrical Conductivity of Glass (http://talk.craftweb.com/showthread.php?p=97697#poststop) And propose that monitoring for conductive molten glass in the bottom of a furnace after a catastrophic pot failure could be easily accomplished by inserting two kanthal wires through frax blocking the cleanout port as shown by this demo (http://www.youtube.com/watch?v=ee9bj4mhosY) . For safety reasons, this can be powered by an isolated low voltage bell transformer powering a doorbell or relay just as well as by line voltage used in the video.

Just keep the wires separated from each other, and off the floor at a height sufficient to clear "normal" debris that you clear in your regular good housekeeping ritual.

For more than you ever wanted to know about conductivity of melted glass:
here (http://glassproperties.com/resistivity/ElectrResistMeasurement.htm)

Steve Stadelman

02-16-2012, 10:43 PM

Great idea, however, the kanthal wire will soon decay in the furnace atmosphere. Even if it's a1

Tom Clifton

02-16-2012, 10:56 PM

They would be easy to change, but if you have a roll of platinum laying around that might last longer...

Dan Ellis

02-17-2012, 12:28 AM

What about tungsten welding rods? That stuff doesn't melt until 3400 degrees, Would they work?

Tom Clifton

02-17-2012, 11:01 AM

What about tungsten welding rods?

Steve - My reply was flip and not thought out as well. For that I apologize.

The whole purpose of the posting is not to provide anybody a ready to commercialize design for the market. Rather food for thought, and a possible means of a ten dollar solution giving a heads up before a catastrophic failure damages a $10k furnace. That may provide sufficent return on investment great enough to warrant overcoming technical limitations of materiels.

Dan - I would believe that anything that is conductive, mechanically rugged and able to survive in a very hostile environment wourld work. So, yes tungsten should do it. I think that .1" dia wire/rod is about a dollar an inch so it is affordable.

Back to kanthal - it is cheap and readily available. For any wire, mechanical support in a mullite tube would be helpful. Likewise a separate "sacrifical" monitoring loop of say 12ga wire would fail before an 8ga sensor wire would. If the circuit on the sacrificial loop broke it could light a "replace sensor" lamp. In the same way a Check Engine light on your car comes on, you have some time to go to the dealer before the oil pressure light conmes on (and presulabley you stop driving immediately).